Meshing Tips & Tricks: General Pre-processing Guidelines For Mechanical & Fluids Users

Meshing Tips & Tricks General pre-processing guidelines for Mechanical & Fluids users

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Topics: • Dealing withover-detailed geometry – When to use CAD tools and when not to – Mesh-based defeaturing (global and local)

• Dealing withspecific meshing requirements – Match/Periodic & Instancing – Thin meshing/Mesh quality – Hex/shell & Mapped mesh control

• Dealing withinterfaces between parts – Shared Topology – Contact

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Dealing withover-detailed geometry and/or sloppy CAD models

• What tools to use in SpaceClaim – Repair Tools – What problems should you fix? Which should you leave alone?

• Mesh-based defeaturing – Roll of local sizes – Roll of pinch controls – Roll of VTs

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SpaceClaim “Repair” Tools

Primarily important

Work top to bottom to stitch sheets to solids, find/fix gaps and/or missing faces

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Helpful for cases where you want a hex or mapped shell mesh or a special meshing need like match meshing For tet meshing defeaturing is often a better choice

Use with caution, especially if you plan to use Shared Topology. Tips: • Avoid altering interfaces between parts • Aggressive use of these tools can lead to other modeling or meshing failures if faces become distorted • Tools are powerful, but requires some experience

When should I repair/simplify the geometry

• Easy answer – “Only when you need to”… But when do you need to?

More constraints 5

• This answer depends on the type of mesh needed for the simulation. There are 3 general types here: 1. Tet/shell mesh 2. Constrained tet/shell mesh Constrained tet or shell mesh: 3. Hex mesh 1. Periodic/match mesh 2. Mapped meshing constraints 3. Thin meshing constraints

Tet/shell mesh guidelines:

Don’t use Geometry repair or Auto-VT or pinch controls unless needed. • No need for geometry repair/cleanup unless: – There are real features cutting the volume that shouldn’t be there – See constrained tet meshing and/or hex meshing sections • Mesh-based defeaturing should be default approach to dealing with over-detailed features in CAD • Worst case, use local mesh sizing, pinch controls or VTs to fix meshing problems

Note:

Proactively fixing meshing problems withrepair or auto-VT can cause more problems than it fixes and is discouraged. Older models can often be simplified by removing all VTs and using mesh based defeaturing instead. 6

Tet/shell mesh guidelines: Mesh-based defeaturing Face Size

Min Size

Notes: • With curvature and/or proximity size function defeature size is set to ½ min size to avoid conflicting constraints where mesher tries to refine mesh to capture curvature/proximity and coarsen mesh to remove features

Min Size

Defeature size

Min Size

• With uniform size function, defeature size is ½ element/face size • With the improvements in tet meshing, patch independent (PI) tet should not be needed anymore.

Mesh-based defeaturing has significantly increased robustness of meshing tools!

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Tet meshing guidelines: Features cutting the volume If there are actual holes or gaps in the volume, the mesher will often not allow such features to be removed automatically: • This is by design as there is concern that cracks, flow obstructions, etc may be missed. • However, there is an exception for seamless cylinders as shown below: Seam (2) faces causing real gap

Exception: Seamless (1) face 8

Tet meshing guidelines: Local size controls Use local size controls on faces/bodies to capture/remove features: • More local sizing takes priority, so smaller defeature size leads to capturing features • Larger defeature size leads to removing features:

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Constrained tet/shell mesh guidelines: Understanding mesh controls

Meshing controls add constraints to meshing, the following constraints/limitations should be understood:

• Match/Periodic controls • Instancing • Mapped mesh controls

• Thin meshing • Hex meshing • Mesh quality

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Understanding mesh controls: Match/Periodic controls Notes: • Linear or Cyclic periodicity automatically creates match controls

• Can’t match across parts, use contact match mesh instead if possible (see next slide)

• Faces can vary a bit and mesh will be adjusted to be periodic

• Need same edge topology on both sides, diagnostics will point out problems as shown below

High side faces are meshed

LCS is not positioned exactly right 11

Low side mesh is matched exactly, but needs to come off geometry to accommodate this

Understanding mesh controls: Contact match mesh Notes: • Only works with tet mesh • Mesh generated as separate parts • Contact regions can be promoted to Contact match controls • Use contact match controls to match the mesh after the fact • Node merge can be used to make mesh conformal (alternate approach to Shared Topology)

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Understanding mesh controls: Match/Periodic controls • Use with instancing to pattern meshes for repetitive features: Use with node merge to get conformal mesh

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Understanding mesh controls: Instancing Notes: • Instancing defined in CAD/SC/DM • Instances need to be defined at part level (can’t span multi-body part)

• A multi-body part can be instanced (e.g. example on right) • If defined, mesh is copied for instanced parts that have same mesh controls, if mesh controls are different, unique parts are meshed (not copied) • Use copy to instances for easier setup and to better understand instancing • ACT extension to help set up instances

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Understanding mesh controls: Instancing & Match Control Video Demo

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Understanding mesh controls: Mapped mesh controls Notes: • Use with tet mesh to get split-tri pattern – Can use withcriterion based selection to get cylinders – Does not work with defeaturing so use selectively

• Use with hex mesh to enforce mappability of side faces, or to enforce free mesh

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Understanding mesh controls: Thin meshing Isotropic thin meshing:

Anisotropic thin meshing:

Good solution with proximity SF:

Multiple approaches:

• Refines mesh to put defined number of cells in gap

• Due to size concerns, some customers want to use anisotropic mesh in the thin regions. Approaches:

• Can significantly increase element count, but in many cases this is necessary for simulation accuracy 17

– Use inflation for thin meshing – Slice out thin region and use hex meshing – MultiZone

Understanding mesh controls: Anisotropic Thin Meshing Inflation:

Split & Thin Sweep:

MultiZone:

• Use local inflation controls to • Split out thin region; may want • Split out thin region; create control height through thickness to leave thin region as separate multibody part part • Set gap factor to small value • Supports program controlled inflation and multiple elements • Element quality in middle could • Thin sweep only allows multiple elements across thickness for through thickness be bad… single body parts, could use MultiZone instead 18

Understanding mesh controls: Hex meshing Notes: • Which strategy should I use? • Which method should I use? – Sweep – Thin Sweep – MultiZone

• Trade-offs to consider: – – – –

Source/target handling Defeaturing Inflation Mesh quality

• Several diagnostics tools available

• MultiZone tips & tricks

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Understanding mesh controls: Hex meshing Which strategy should I use?

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1. Use power of SpaceClaim to quickly slice everything into sweepable bodies

2. Use auto-decomposition in MultiZone to decompose regions

When to use:

When to use:

• When model is mostly regular with faces aligning to planes (easy to slice)

• When model is irregular with curved faces that need to be swept in difficult fashion

• Not too many small features that can create problems in Shared Topology

• Not too many small features that can create problems in Shared Topology

Understanding mesh controls: Hex meshing  Sweepable bodies 1. Use power of SpaceClaim to quickly slice everything into sweepable bodies Approach:

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It is often easier to slice, then to set up mesh methods specifying source/target faces and/or edge sizing controls for proper edge distribution.

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Leverage the automation of sweeping by slicing the model so that the source/targets are easy for software to determine.

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This approach will often have MANY bodies, however, the meshing should be very automatic so it shouldn’t matter. Make sure splits run entirely through the model so that parallel edges are easy for the mesher to identify and ensure same number of elements are defined on parallel edges. Good!

Understanding mesh controls: Hex meshing  MultiZone 2. Use auto-decomposition in MultiZone to decompose regions Approach: •

Minimize slicing to avoid small edges that can create problems in shared topology

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Leverage the auto-decomposition in MultiZone to automate the internal slicing and shaping of internal edges/faces

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Use face splits rather than body splits to allow MultiZone to shape interiors

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Use inflation for further decomposition

Circular internal loops on side faces submapped With inflation, good quality mesh is achieved on circular internal loops

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Understanding mesh controls: Hex meshing methods

Which method should I use? • While first and second strategies generally align with Sweep and MultiZone respectively, a combination of the 2 mesh methods can also help. • Be aware, however, that Sweep and MultiZone behave slightly differently as do their own interval edge assignments, so whatever method is used in later steps need to get the mesh spacing from the previously meshed parts (see next slide). • Following slides provide some guidelines on differences between methods

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Understanding mesh controls: Hex meshing step-wise Bodies on left and right are meshed via selective meshing:

Essentially the same setup as this:

Middle body then fails to mesh, why?

Interval (parallel) edge assignment notes: •

For sweeping to be successful side faces have to be mapped, thus parallel edges need to get the same number of divisions

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Use hard sizing to help selective meshing. • In case above, if user used hard edge sizings to force the left and right edges to be the same, selective meshing would work.

•

Only use selective meshing & hard edge sizing when required: • Pre-meshed edges can not be changed and thus are essentially hard. • Setting hard edge sizes over-constrains the mesher creating difficulties to get proper mapped mesh on side faces, so use with caution.

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Understanding mesh controls: Hex mesh source/target handling Method

What are the sources/targets?

Side faces?

Notes:

Sweep

Single source to single target. Specifying the source/targets defines the sweep path.

Side faces need to be mappable, but supports multiple levels of side faces

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It is some times necessary to set the source or source and target so that things sweep in the desired fashion. Examples: • Sweep radially into wedge • To ensure proper meshing order • Can have multiple levels of side faces as long as they are all mappable

Thin Sweep

Multiple source faces to multiple target faces.

Side faces should directly connect source and target faces, if not use VTs to merge the side faces

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Automatic thin should work for cases where a user could select one source face and flood select to all source faces. For cases where there is a sharp feature preventing the flood fill, the user should use Manual thin. Faces attached to the boundary of the manual source faces are side faces All other faces are treated as target faces

Side faces need to be mappable

•

MultiZone

Multiple source to multiple target faces.

• •

• •

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Does not distinguish between source and target faces so both sets of faces should be selected as source faces. Other faces are treated as side faces Intersecting imprints are not supported. User should slice geometry to avoid this. Software tries to submap side faces to get swept mesh, but guide-curves are often helpful if the software struggles in multi-level cases.

Understanding mesh controls: Hex mesh defeaturing Method

What topology has to be respected

Notes:

Sweep

Everything: All vertices, edges, faces of source, target and side faces

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User needs to clean topology they are not interested in either in CAD or by using VTs

Thin Sweep

All source faces are respected. Target faces can be ignored, provided side faces are clear. In example below, the mesh will fail due to the split in the side face:

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Good for single body parts: • only 1 element through thickness for multibody parts • For stack of parts user needs to control meshing order as target faces are also used as source faces The thickness is really a factor of mesh/feature size:

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MultiZone

Option to respect all topology, or protected topology (BCs, NSs, etc)

•

•

Defeature size to ignore slivers 26

MultiZone generally respects sharp features and will ignore flat features unless topology is protected. Protected topology is defined as objects that are scoped to a named selection, boundary condition, etc. • Setting a defeaturing tolerance can increase the amount of defeaturing done. • Setting Preserve boundaries to All will stop all defeaturing. MultiZone is often the most “robust” approach for dirty geometry as it simply skips some of the constraints.

Understanding mesh controls: Hex mesh inflation Method

How to define inflation

Notes:

Sweep

Need to use Manual source and use 2D inflation on source face

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2D inflation with Sweep is supported, but whenever inflation is needed it is generally easier to use MultiZone.

Thin Sweep

Inflation not supported

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Use other method if inflation is required

MultiZone

MultiZone supports 3D inflation so select body and faces to inflate. Can also be used with program controlled inflation

• •

Inflation setup is similar to tet meshing Use inflation to improve quality in regions like below:

Circular internal loops on side faces submapped

With inflation, good quality mesh is achieved on circular internal loops

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Understanding mesh controls: Hex meshing diagnostics Notes: • If something looks sweepable, but is not found to be sweepable, often there is something strange with the topology Use the following tools to diagnose such problems:

• – – – – –

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Show sweepable bodies Show vertices Color edges by connectivity Show mappable faces Final check is to mesh it!

Splitting edges can show seamless edge start/endpoint

Understanding mesh controls: Mesh Quality Notes: • Mesh criteria limits are subjective, so don’t waste too much time… – Criteria differ based on physics and solver – Often more useful to look at mesh sensitivity studies or error measurements

• Use target quality to set target limits • Avoid Adaptive Size Function • If only a few bad elements they can be fixed via mesh editing

Criteria

Pictorial

Bad

Good

Excellent

Element Quality

Combination of below

0.001-0.10

0.20-0.69

0.95-1.00

Aspect Ratio

>10

10-5

5-0

Warping

>0.4

0.4-0.2/0.1

0.2/0.1-0

Jacobian

>30

30-10

1

Skewness

>0.75

0.75-0.25

0.25-0

Criteria for ANSYS Structural solvers 29

Understanding mesh controls: Mesh Quality A good looking mesh is not necessarily a better mesh

Higher is better

Lower is better 30

Understanding mesh controls: Mesh Quality Performing a mesh sensitivity study on local or global mesh sizing can help identify optimal values

Refining the mesh drastically reduces the local error in the fillet area

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Understanding mesh controls: Mesh Quality Comparing averaged and non-averaged results such as stresses is an indicator of the local quality of a mesh

Comparing averaged vs. non-averaged results can indicate if mesh quality is sufficient. If results are comparable, it indicates a sufficient mesh density 32

Understanding mesh controls: Mesh Quality

Mesh Size

Feature Capture

Small collar Mesh Quality

Small fillet

Meshing is often an over-constrained problem…

Adaptive SF

• What is the right mesh?

Mesh Size

2 approaches:

Feature Capture

1. Give priority to features Mesh Quality

2. Give priority to mesh size/quality Curvature SF

Mesh Size

Mesh Quality 33

Feature Capture

Adaptive SF is good for getting a coarse mesh that captures all features. Other SFs give higher quality mesh, and will either skip or refine mesh to capture features (see defeaturing).

Dealing withinterfaces between parts

• Advantages of Conformal vs. Non-conformal meshing • Shared Topology – New options in SpaceClaim 18.1 – Diagnostic tools in Meshing/Mechanical to ensure proper connectivity

• Contact Detection – Automatic contact detection – Contacts automatically transferred as interfaces for Fluent – Cyclic redundant contact regions

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Advantages of Contact/Non-conformal interfaces Conformal approach:

Non-conformal approach:

• Connected mesh can often lead to better or faster solution:

• Non-conformal mesh can often lead to faster meshing times:

– No need for interpolation across mesh boundaries – May more accurately represent physics

• Mesh sizing at interface takes sizing of both bodies into account Notes: • General approach to get Conformal mesh is to first use Shared Topology to get conformal geometry.

• Alternative approach can be to use nonconformal meshing with matched nodes and then merge nodes to make the mesh conformal 35

– Doesn’t required Shared Topology / Mesh – Part meshing is generally faster (parallel part by part) – Sizing constraints at interface simplified (can be significant for hex meshing). Can also reduce the element count.

• In some cases more appropriately represents the physics (Contact, Sliding interfaces, etc.)

Shared Topology: New options in SpaceClaim 18.1

Notes: • Improved robustness and performance (more work ongoing) • Improved display and diagnostics options • Local tools to fix problems, unshare, etc. • If you want to make CAD changes (Pull, Move, etc) after Sharing, best to Unshare everything, make the change, and then reShare

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Shared Topology: Local sharing/unsharing:

• Selectively share or unshare Share selected regions

Exclude middle faces

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Shared Topology: Understanding part structure

No Share Topo:

With all bodies shared:

Single parts

One multibody part

Software finds the connectivity and forms multibody parts as appropriate

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Shared Topology: Diagnostic tools to check for proper connectivity Tools to check connectivity: • Color by connectivity shows edge connectivity • Check contacts to find faces in proximity that are not shared • Use close vertices to detect small gaps

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Contact Detection: Automated checks for interfaces between parts Notes: • By default, contact detection is done on attach (model import). It is recommended to do this, but a user should check on the default values as often they will be too large: • As mentioned, contact can indicate problems with Shared Topology • If tolerance is too large, unnecessary contacts can be created, this can bond faces or create interfaces for Fluent that are not needed. • It is good to validate that automated contacts represent the engineering intent for the model.

• It is a good practice to use Tolerance Type: Value; setting a reasonable tolerance for the gap between parts, and regenerating the contacts. • Setting the face overlap tolerance to 50% is a good practice for Fluids users or if you want to reduce the contact area

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Contact Detection: Face Overlap Tolerance • New option at 18.1 – Used to reduce contact areas to just those areas that overlap within the defined tolerance. – Default for Mechanical is Off (same as in past)

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Contact Detection: Matching boundaries via imprint Notes: • Use imprint to get similar boundaries between parts. Faces will still be duplicate (non-conformal mesh), however boundaries should be similar. • This is particularly important for Fluent as Fluent doesn’t support cyclic redundant contact/interfaces (see next slide)

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Without the imprints, the contact region mesh looks like this:

Contact Detection: Cyclic redundant contacts/interfaces in Fluent Cyclic redundant case

Notes: •

Cyclic redundant contact occurs when the source/target (master/slave) relationship is over-constrained A is master of B, B is master of C, C is master of A…

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This can happen when mesh regions are not properly separated to form unique pairs of interfaces. Imprinting or splitting the faces solves the problem.

•

Use check overlapping contact regions to find faces that are in multiple contact regions:

A B C

• A face in multiple contact regions may not be a problem, but it may indicate a problem with cyclic contacts/interfaces

Cyclic redundant case: Because the faces between boxes aren’t split and both touch air and other box, the interfaces are cyclic. The simple fix is to imprint! 43

Contact Detection: Matching sizes Notes: • Use contact sizing to ensure similar mesh sizes between source and target faces • Contact sizes can be generated by dragging/dropping the contacts onto the mesh folder

• By default, smaller size is used, or user can define a size for the interface

without contact sizing

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with contact sizing

Contact Detection: Diagnostic tools Notes: • Since understanding contact regions is some times hard to visualize there are some useful techniques in addition to the body views to better understand the contact areas: • Use Promote to Named Selection to get a named selection from the source entities and target entities • If trying to understand contact between 2 bodies, use group by Faces to see 1 to 1 pairing between faces rather than grouping the faces into 1 contact region.

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Other tips to improve productivity:

• Mesh status • Model assembly

• Using folders

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Meshing Status: Notes: • Mesh status bar reports performance of each part as it is meshed in parallel (if you don’t see multiple lines it’s not doing parallel part by part meshing) • The highlight option is good for tet meshing to show which object the mesher is working on. • Can be toggled off • If a face is highlighted for a long time it could indicate problems with that face.

• If meshing is stopped, completed parts stay meshed. Generate mesh will re-start with unmeshed parts.

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Model Assembly: +

+

=

Shock-Right

Shock-Left TieMechanism

Notes:

Suspension

Suspension

• Use model assembly to simplify setup • Models can be meshed and updated independently • Transformations and copies can be used • Filtering and grouping helpful to quickly organize data

+ Left Wheel

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+ Suspension

Right Wheel

Folders for Meshing Controls Notes: • Group All Similar Children will group all objects based on type • Options to suppress, rename, nest groups, ungroup, delete objects in group

• Drag and Drop • Often easiest to group by part • Use with filtering to narrow in on problems

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Thank you for your time

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